This project deals with a research model for testing the abuse potential of drugs. The model, approach conditioning, uses the test drug to reinforce rats for entering a compartment. It measures reinforcement by speed of entering the drug compartment, by choice of that compartment over a nondrug compartment, and, in one experiment, by the rate of a response that gains access to the drug compartment. The drugs are given by intravenous infusion in five experiments, and by intraperitoneal injection in one. The proposed research will investigate several possible ways to make approach conditioning more sensitive to differences in the reinforcing efficacy of different doses of morphine, heroin, and meperidine: decreasing the number of conditioning trials, allowing the subject to express a choice between two doses, using the measure of response rate, and measuring changes in behavior when doses are changed. It will also assess the ability of the speed and rate measures to trace the process of conditioning and changes in drug-seeking behavior after conditioning.